The invention relates to acidizing a subterranean formation and, more particularly, to a new and improved method for acid fracturing a formation surrounding a wellbore.
It is common practice to acidize subterranean formations in order to increase the permeability thereof. For example, in the petroleum industry it is conventional to inject an acidizing fluid into a well in order to increase the permeability of a surrounding hydrocarbon-bearing formation and thus facilitate the flow of hydrocarbon fluids into the well from the formation or the injection of fluids, such as gas or water, from the well into the formation. Such acidizing techniques may be carried out as "matrix acidizing" procedures or as "acid fracturing" procedures.
In matrix acidizing, the acidizing fluid is passed into the formation from the well at a pressure below the breakdown pressure of the formation. In this case, increase in permeability is effected primarily by the chemical reaction of the acid within the formation with little or no permeability increase being due to mechanical distortions within the formation as in fracturing.
In acid fracturing, the acidizing fluid is disposed within the well opposite the formation to be fractured. Thereafter, sufficient pressure is applied to the acidizing fluid to cause the formation to break down with the resultant production of one or more fractures therein. An increase in permeability thus is effected by the acid etching of the fractures formed as well as by the chemical reaction of the acid within the formation.
In yet another technique involving acidizing, the formation is fractured initially using an inert fluid. (Inert fluids may be gelled water containing polymers such as hydroxypropyl cellulose). Thereafter, an acidizing fluid is injected into the formation at fracturing pressures to extend the created fracture and to "finger through" the fracture filled with the inert fluid. The acid functions to dissolve formation materials forming the walls of the fracture, thus increasing the width and permeability thereof.
In A. R. Jennings, Jr. U.S. Pat. No. 4,807,703, there is described an acid treatment of a subterranean formation to improve productivity of hydrocarbons from a reservoir therein employing an acid fracturing of the formation surrounding a well pentrating the hydrocarbon reservoir. Such acid treatment involves the injection of a gelled and foamed acid into a formation under conditions and pressures sufficient to fracture the formation and create a foam pad in the face of at least one resultant fracture. Thereafter, an ungelled and foamed acid is injected into the fracture which acid "fingers" through the foam pad thereby unevenly etching the fracture and propagating to substantially further distances into the formation. When the acid is spent and the desired amount of etching and fracturing is obtained, the gelled composition is removed from the formation. Hydrocarbon fluids are then produced from the formation.